Electric motor



May 24, 1938- c. o. BERGSTROM ELECTRIC MOTOR Original Filed Nov. 27,1934 Patented May 24, 1938 UNITED STATES.

PATENT OFFlCE- ELECTRIC MOTOR Carl 0. Bergstrom, Boston, Mass assignor'to B. F. Stnrtevant Company, Hyde Park, Boston,

Mass.

Application November 2'1, 1934, Serial No. 754,991 Renewed January 5,1937 4 Claims.

This invention relates to alternating current motors of the type havinga revolving, magnetic field.

In the ordinary induction motor, a rotating, magnetic field is set up,which revolves at synchronous speeds, the revolutions per minute ofwhich are equal to frequency times sixty, divided by the number of pairsof poles. The armature or rotor has short circuited, secondary windings,across the conductors of which the revolving, magnetic field passes andinduces in these secondary conductors, currents which, according to theLenzs law, are in such direction as to oppose the cause. That is to say,these secondary currents react on the revolving, magnetic flux in such away as to draw the secondary conductors and associated rotor structurealong with the retating flux. The armature or rotor of the ordinaryinduction motor can never obtain full synchronous speed, there alwaysbeing a slip, causing the maximum armature speed to be a considerablenumber of revolutions per minute lower than the synchronous speed of thefield.

According to a feature of this invention, a motor is provided, thearmature of which revolves at speeds greater than the synchronous speedof its revolving field. lhis is accomplished by providing a field orstator structure in which the length of travel of the rotating flux issubstantially greater than the circumference of the ar mature, with theresult that, while its revolutions per minute are unchanged over that inthe ordinary stator, its speed at any point, in feet per second, isconsiderably increased with corresponding increase in the number ofrotor conductors cut per unit of time by the flux, andv correspondingincrease of revolutions per minute of the rotor. Thus, the speed of therotor, neglecting for the moment the usual slip. is increased an amountproportional to the length of travel of the magnetic flux in onecomplete circuit of the field, divided by the circumference of therotor.

According to another feature of this invention, the speed of the rotormay be easily varied from speed substantially equal to the synchronousspeed of the field to a predetermined maximum. This is accomplishedby-varying the length of travel of the revolving, magnetic flux.

An object of the invention is to increase the speed of the rotor of amotor having a revolving field above that of the synchronous speed ofthe field.

Another object of the invention is to vary the speed of the rotor of amotor having a revolving field, by varying the length of travel of thefield flux.

The invention will now be described with reference to the drawing, ofwhich:

Fig. 1 is a diagrammatic view of one embodi: 5 ment of the invention inwhich a rotor of an induction motor is mounted off-center in arelatively large stator;

Fig. 2 is a diagrammatic view of another embodiment of the invention inwhich the stator 10 has but slightly greater diameter than the rotor,except for extensions at the two sides thereof Fig. 3 is a diagrammaticview similar to Fig. 2, except that one of the stator extensions isprovided. with a movable, magnetic shunt for varying the length oftravel of the magnetic flux;

Fig. 4 is a diagrammatic view similar to Fig. 3, except that both statorextensions are provided with movable, magnetic shunts;

Fig. 5 is a diagrammatic view of another embodiment of the invention,the stator being divided into two portions, one of which follows ciosel;the outline of the rotor except for a small gap at one side, the-otherportion being circular in form and having a revolvable, magnetic shunt;25

Fig. 6 is a projected, diagrammatic View of another embodiment of theinvention, which is similar to that illustrated by Fig. 3, except thatthe rotor is divided into two spaced portions and the stator is dividedinto two cooperating spaced portions to prevent the leakage of fluxacross the gaps between the extensions of the stator;

Fig. 7 is a side view of Fig. 6. on reduced scale,

and

Fig. 8 is a sectional view along the lines 8-8 of Fig. '7.

In the ordinary i duction motor, the rotor is dragged, so to speak,around by the rotating magnetic flux in the stator. Ordinarily, thestator structure, in efilciencys interest, follows closely the outlineof the rotor, the two being separated by a very small air gap. Since therotating, magnetic flux in the stator drags at the surface of the rotor,the rotation of the latter is caused by a surface effect. So, when therotor 45 iii is displaced off-center in a relatively large stator Ii, asillustrated by Fig. 1, the magnetic flux passes each of the conductorsof the rotor at a considerably higher surface speed than if the rotorwas centered in the stator. The larger the stator for a given size rotorin an off-center arrangement, as shown by Fig. 1, the greater the speedof revolution of the rotor, this because, although the synchronous speedin revolutions per minutemay remain the same, the linear or fashion, oronly that portion adjacent and oppo-' as indicated by Fig. 1, may besitethe rotor, wound.

Realizing that, while with the arrangement illustrated by Fig. 1,increased speeds may be attained by undesired loss 01' efilciency, thearrangements of Figs. 2 to 8 inclusive provide a more efiicient magneticstructure.

With reference to Figs. 2 to 4 inclusive, the rotor I0 is provided witha stator l l which follows closely the outline of the rotor, except forthe two extensions l2 and I3. Thus, the air gap between the rotor andstator may be that in usual practice, except for the two extensions ateach side 01' the stator. These extensions give an increased travel forthe magnetic flux of the field, with the result that with eachrevolution of the field flux, it cuts a larger number oi conductors of arotor per unit of time than it would were the extensions not provided,with the result that the rotor revolves at speeds above the synchronousspeed of the field, this increased speed being substantially equal tothe synchronous field speed multiplied by the length of travel of thefield fiux, divided by the circumference of the rotor.

In the arrangement shown by Fig. 3, the extension l3, instead of beingclosed, has a movable, magnetic shunt I4, which may be moved towards oraway from the rotor to vary the length oi travel of the fiux of themagnetic field. The magnetic flux in each case passes across the magnetic shunt i4. Thus, the position of the shunt I 4 determines thelength or travel of the flux.

Fig. 4 illustrates an arrangement similar to Fig. 3, except that thestator extension H has a magnetic shunt l5 similar to that of themagnetic shunt l4 oi the stator extension l3. When the two shunts l4 andI5 are moved to their innermost position nearest the rotor, the rotorwill obviously revolve at the synchronous speed of the field minus theusual slip. When either or both of the magnetic shunts are moved awayfrom the rotor, the speed of the latter increases propor-.

tional to the increase in the length of travel of the magnetic fluxcaused by the positions of the magnetic shunts.

Fig. 5 illustrates another arrangement for ob-- taining longer fieldfiux travel. In this arrangement, the stator II is provided with acircular extension l6, on which are rotatably mounted the two magneticshunts l1 and I8, which may be moved together towards the rotor I ll toshorten the length of travel of the magnetic field flux,

or oppositely to increase the length of travel of the revolving fieldflux. Thus, the speed of rotation of the rotor Ill may be increased orde-' creased by varying the position of themagnetic shunt formed. by themembers H and 58.

Since under some conditions, dependent upon the nature of duty, magneticmetals employed, etc, there might be appreciable leakage across the gapsadjacent the rotor, between the upper and lower members or" theextension id or ex tension iii, the embodiment of the inventionillustrated by Figs. 6, '7 and 8 is provided with two rotors l9 and 2d,the stator being formed in two off-set portions, 2i cooperating withrotor 2d, and E22 cooperating with rotor ii). The stator is pro-= v inand out towards the rotors to decrease their amount of magnetic flux mayleak across the 5 gap, substantially all of the working fiux taking thepath defined by the magnetic material forming the stator, itsextensions, and its magnetic shunt 25. The magnetic shunt 25 may bemoved speed of rotation, or away from the rotors to increase their fieldof rotation.

The leakage across the gaps referred to in the preceding paragraph mayalso be reduced or prevented by providing the rotor with deeper slotsthan are usually employed. This willincreaso the core density, providinga higher leakage reactance, with the result that the magnetic flux willtake the path of lower reactance through the stator structure.

It has been determined that, according tothil invention, high speedinduction motors may be provided. The limiting factor in rotor speedheretoiore has been the number of field poles and frequency available.High speed induction motors have, therefore, not been available.'Thisinvention, therefore, provides an induction motor, the, speed ofwhich may be increased far above that now ordinarily obtained. Such highspeeds are desirable even though obtained, with slight loss ofefllclency. I

While the invention has been illustrated as applied to an inductionmotor with single phase windings andsquirrel cage rotor, it should beunderstood that the invention should not be so limited, but may includeinulti-phase motors of any type which employ rotating magnetic fields,and rotating field motors with squirrel cage type or wound rotors.

Whereas several embodiments of the invention have been described 101'the purpose of illustration, it should be understood that the inventionis not limited to the details described, since many modifications may bemade by those skilled in the art without departing from the spirit ofthe invention.

What is claimed is:

1. An induction motor of the type having a revolving magnetic field fluxcomprising a magnetic rotor, a stator around said rotor having a plural-,ity of portions through which the revolving field flux travels aroundsaid rotor, one of said portions of said stator being closely spacedfrom the surface of said rotor and another portion of said stator beingspaced a substantial distance from said rotor, and field windings eachhaving a plurality of turns, on said stator on opposite sides of, saidrotor for producing the magnetic field.

2. An induction motor of the type having a revolving magnetic field fiuxcomprising a magnetic rotor, and a stator around said rotor having aplurality of portions through which the rotating magnetic fiux aroundsaid rotor travels, one of said portions being so closely spaced fromthe surface of said rotor as to provide a small air gap between saidstator and rotor and another of portions being so spaced from thesurface of said rotor as to provide substantial air gap between asubstan a]. surface of said stator and said rotor where y the ion tn ortravel oi the revolving field fit... in sold stator is aiiy than theclrcrun rence of said rotor.

An induction motor or the type having revolving magnetic fie co 2. neticrotor, and a Mount. Mac. rotor hav ing a plurality of portions throughwhich the rotating magnetic flux around said rotor travels, one of saidportions being so closely spaced from the surface of said rotor so as toprovide a small airgap between said stator and rotor and another of saidportions being so spaced from said rotor as to provide so substantial anair gap between a substantial surface of said stator and said rotor thatthe revolving field flux can have substantially no rotative effect uponthe surface of the rotor adjacent same.

4. An induction motor of the type having a revolving magnetic field fluxcomprising a magnetic rotor, a stator around said rotor having aplurality of portions through which the rotating magnetic flux aroundsaid rotor travels, one of said portions being'so closely spaced fromthe surface of said rotor as to provide a small air gap between saidstator and rotor and another of said portions being so spaced from thesurface of said rotor as to provide a substantial air gap between asubstantial surface of said stator and said rotor whereby the length oftravel of the' revolving field flux in said stator is substantiallygreater than the circumference of said rotor, and

field windings each having a plurality of turns,

producing the magnetic field.

CARL O. BERGSTROM.

